1. Field of the Invention
The invention relates to the use of medium to high voltage load interrupter switches or circuit interrupters with metal resistors having a positive temperature coefficient of resistivity (PTC elements).
2. Background of the Art
Circuit breakers and circuit interrupters or interrupter switches are widely used in residential and industrial applications for the interruption of electrical current in power lines upon conditions of severe overcurrent caused by short circuits or by ground faults. One of the problems associated with the process of interruption and breaking of the current during severe overcurrent conditions is arcing. Arcing occurs between the contacts of circuit interrupters used to break the flow of electric current in a line, which is highly undesirable for several reasons. Arcing causes deterioration of the contacts or blades of the circuit interrupter and causes gas pressure to build up. Arcing also necessitates circuit interrupters with larger separation between the contacts in the open position to ensure that the arc does not persist with the contacts in the fully open position.
Prior art devices have used a number of approaches to limit the occurrence of arcing. In heavy-duty interrupter switches, the contacts may be enclosed in a vacuum or in an atmosphere of SF.sub.6. Both of these approaches are expensive and, SF.sub.6 has been identified as environmentally undesirable.
Another approach that has been used to limit the amount of arcing is the use of a resistor connected in parallel with the main contacts of the circuit interrupter. Upon opening of the main contacts, current can still flow through the shunt resistor, effectively reducing the amount of arcing in the main contacts. The current flowing through the resistor is less than the short circuit current that would flow through the main contacts in the absence of the resistor, and the opening of a second pair of contacts connected in series with the resistor can be accomplished with less arcing than would occur in the absence of the shunt resistor.
Tanaka et al., (U.S. Pat. No. 5,424,504), teach a circuit breaker in which a resistor-provided UHV breaker has a tank sealing an insulating gas, a main contact and a resistor unit connected in parallel to the main contacts also located in the tank. Mechanisms are provided so that the resistor contact is made before and broken after the main contact is made and broken. The resistor has to be rated to withstand the high currents and temperatures during short circuit conditions.
Khalid, (U.S. Pat. No. 4,070,641), teaches a current limiting circuit breaker in which the current limiting contacts are in series with the main contacts of a breaker. Opening of the limiting contacts shunts high fault current through the resistor. The resistor is an iron wire resistor with a positive temperature coefficient of resistance. The flow of the short circuit current through the resistor heats the resistor, thereby increasing its resistance and limiting the buildup of the short circuit current.
Chen (U.S. Pat. No. 5,629,658) discloses a number of devices in which PTC elements are used in conjunction with two or more switches to limit the current under short circuit conditions and thereby reduce the associated arcing.
The present invention achieves the breaking of an electric current in a line with a reduction in arcing, noise and gas venting. The circuit interrupter and method of the present invention increases the switching capacity for a high voltage load circuit interrupter especially for inductive circuits with a power factor lower than 25%. The present invention reduces the time for breaking the flow of electric current by increasing the arcing blade path resistance and the power factor of the circuit while also reducing the cost of the circuit interrupter.